db: integrate golang-migrate for schema migrations

- internal/db/migrations/: five embedded SQL files containing
  the migration SQL previously held as Go string literals.
  Files follow the NNN_description.up.sql naming convention
  required by golang-migrate's iofs source.
- internal/db/migrate.go: rewritten to use
  github.com/golang-migrate/migrate/v4 with the
  database/sqlite driver (modernc.org/sqlite, pure Go) and
  source/iofs for compile-time embedded SQL.
  - newMigrate() opens a dedicated *sql.DB so m.Close() does
    not affect the caller's shared connection.
  - Migrate() includes a compatibility shim: reads the legacy
    schema_version table and calls m.Force(v) before m.Up()
    so existing databases are not re-migrated.
  - LatestSchemaVersion promoted from var to const.
- internal/db/db.go: added path field to DB struct; Open()
  translates ':memory:' to a named shared-cache URI
  (file:mcias_N?mode=memory&cache=shared) so the migration
  runner can open a second connection to the same in-memory
  database without sharing the handle that golang-migrate
  will close on teardown.
- go.mod: added golang-migrate/migrate/v4 v4.19.1 (direct).
All callers unchanged. All tests pass; golangci-lint clean.

internal/db/migrations/000003_failed_logins.up.sql

@@ -0,0 +1,8 @@
+-- Track per-account failed login attempts for lockout enforcement (F-08).
+-- One row per account; window_start resets when the window expires or on
+-- a successful login.  The DB layer enforces atomicity via UPDATE+INSERT.
+CREATE TABLE IF NOT EXISTS failed_logins (
+    account_id    INTEGER NOT NULL PRIMARY KEY REFERENCES accounts(id) ON DELETE CASCADE,
+    window_start  TEXT    NOT NULL,
+    attempt_count INTEGER NOT NULL DEFAULT 1
+);